The present invention relates generally to well logging recorders. More particularly, the present invention is directed to a method and apparatus for controlling an electrophotographic well logging recording device for improving the clarity of the recordings and to eliminate artifacts and other undesirable markings on the recording. The method and apparatus of the present invention provide for imaging and processing of electrophotographic film, paper or other recording medium in real time over a wide range of throughput speeds.
Electrophotographic processes typically use four operational steps, namely charging, exposing, adding toner, and fusing the toner to electrophotographic recording medium transported between various stations in an electrophotographic processing unit.
The recording medium generally used is a three layer film such as Eastman Kodak type SO-101. That film has a bottom transparent polyester base similar to that used in conventional film. The center layer is a thin conductor which is electrically connected to a metal spool on which the film is wound. The top layer is a photoconductive dielectric. Apart from the light-sensitive properties of the film, the film, in effect, operates as a parallel plate capaciter with one plate missing. If kept away from light, the film can accept and hold a charge for long periods of time. On exposure to light, the photoconductive layer becomes conductive, and the surface charge of the film is reduced.
In general terms, the film is first sensitized by charging the film surface to a potential of approximately +500 to +600 volts. The charging is typically performed by a corona charging unit consisting of a small diameter wire enclosed on three sides by a conductive metal shell. When a high voltage (4-7 kilovolts) is applied between the wire and the shell, the air surrounding the wire is ionized. That results in a flow of ions from the wire to the shell and from the wire to the film surface. That flow of ions charges the surface of the film to the desired potential.
Other recording mediums, of course, are used, and the method and apparatus of the present invention may be used with such other mediums. For ease of explanation, however, the following description will simply refer to the recording medium as a film.
The second step in a typical electrophotographic process is to expose the film to light in the areas where an image is desired. In electrophotographic techniques used in recording operations, the film is typically exposed by passing it over a fiber-optic face plate of a cathode ray tube (a "CRT"). A CRT with fiber-optic face plate allows selective exposure of the film, and results in a latent electrostatic image. The areas of the film that are exposed become conductive, and the surface voltage in those areas is reduced. The discharge path is through the center conductive layer of the film and the film spool to ground. The surface voltage in the exposed areas is thus reduced below that of the surrounding areas from the previously charged level of +500 to +600 volts to 250-300 volts.
The third step is to develop the film by selectively depositing toner particles in the exposed areas, namely those areas with the lowest surface voltage. In typical electrophotographic devices, the film is passed over a toner head which supplies positively charged toner particles to the film surface. Devices using liquid toner typically suspend toner particles in an insulating or dielectric liquid carrier which is pumped through the toner head from a reservoir. The positively charged toner particles are attracted to the exposed areas, and are repelled from the unexposed background areas. After passing over the film surface, the liquid toner returns to a reservoir via a sump generally surrounding the toner head. An air knife is usually used to strip excess liquid from the film as it leaves the toner head.
The last step of the typical electrophotographic process is to render the image permanent by fusing the toner particles to the film. After toning, the image is visible but not permanent, and can be easily smeared or removed. The fusing process heats the toner particles to a point where they are partially melted into the film surface. The toner particles are generally heated by absorption of infrared energy radiated by an infrared lamp.
The film is finally driven through the electrophotographic recorder by a drive mechanism usually consisting of a drive roller driven by a stepper motor, thus allowing the film to be advanced in steps, typically 0.005" per step, with each step synchronized with the scan of a beam across the CRT face plate.
Well logging operations, however, impose additional requirements on an electrophotographic recorder system. The system must be capable of operating over a wide range of speed (for example, 0.005" per second to more than 1" per second). It must produce good image quality without extraneous artifacts (namely, undesired smears, blotches, and other marks) when the film motion is stopped for extended time periods up to or exceeding one hour. It must be capable of normal operation when tilted 10.degree. or more from a horizontal plane and be capable of normal operation over a wide temperature range (for example, 0.degree. C. to +45.degree. C.). It must also be capable of asynchronous operation meaning that the film must be capable of being moved intermittently in addition to uniform rates and be capable of producing long recordings up to hundreds of feet in length with the additional ability to use either transparent film or opaque paper.
Thus, it is a first general object of the present invention to provide a method and apparatus for producing electrophotographic well log recordings meeting those rigorous requirements.